Dual hemispherical collectors

ABSTRACT

A system and method for collecting radiation, which may be used in a lithography illumination system. The system comprises a first surface shaped to reflect radiation in a first hemisphere of a source to illuminate in a second hemisphere of the source; and a second surface shaped to reflect radiation in the second hemisphere of the source to an output plane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priority toU.S. patent application Ser. No. 10/394,412, filed Mar. 20, 2003, nowU.S. Pat. No. 7,034,320 and International Patent Application No.PCT/US03/40315, filed Dec. 15, 2003. The disclosures of the priorapplications are considered part of (and are incorporated by referencein) the disclosure of this application.

BACKGROUND

A microchip manufacturing process may deposit various material layers ona wafer and a photosensitive film or photoresist on the depositedlayers. The process may use lithography to transmit light throughtransmissive optics or reflect light from reflective optics and areticle or patterned mask onto the photoresist, which transfers apatterned image onto the photoresist. A process may remove portions ofthe photoresist that are exposed to light. A process may etch portionsof the wafer which are not protected by the remaining photoresist. Someof these actions may be repeated.

Extreme ultraviolet (EUV) is one form of lithography. A EUV lithographytool may be used to print a pattern on a photoresist with dimensionsthat are smaller than dimensions achieved by other lithography tools.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of an Extreme Ultraviolet (EUV)lithography tool.

FIG. 2 is a cross-sectional side view of an apparatus which may be usedin a Dense Plasma Focus (DPF) electric discharge source.

FIG. 3 is a cross-sectional view of a dual hemispherical collectorsystem, which may be used in a lithography illumination system.

FIG. 4 illustrates a table of values associated with FIG. 3.

FIG. 5 is a cross-sectional view of another embodiment of a dualhemispherical collector system, which may be used in a lithographyillumination system.

FIG. 6 illustrates an example of a hyperbola and ellipse for a shape ofthe second collector in FIG. 5.

FIG. 7 illustrates a method of using a dual hemispherical collectorsystem.

DETAILED DESCRIPTION

Dual hemispherical collectors are described herein with lithographysystems, but the dual hemispherical collectors may be used with otherlight sources for car headlights, movie/video/slide projectors and otherapplications outside of lithography.

EUV Lithography

FIG. 1 illustrates one embodiment of an Extreme Ultraviolet (EUV)lithography tool 100, which may also be called a “lithographic exposuresystem” or a “EUV scanner.” The lithography tool 100 may include a laser102, a laser produced plasma (LPP) source 104, a plurality of condenseroptics 106, a reflective reticle 107 with a pattern, and a plurality ofreflective reduction optics 108.

Other embodiments of the EUV lithography tool 100 may include othercomponents instead of or in addition to the components shown in FIG. 1.For example, instead of a laser produced plasma source 104, thelithography tool 100 may have an electric discharge EUV source 200, asshown in FIG. 2.

The EUV lithography tool 100 may create plasma by using a laser-producedplasma (LPP) source 104 or an electric discharge source 200. Alaser-produced plasma (LPP) source 104 produces plasma by focusing alaser 102 onto a gas, liquid, or filament jet of a material ormaterials, such as Xenon.

An electric discharge source produces plasma by pulsing a currentdischarge (kAs) (like a powerful arc welder) through a gas, such asXenon. The plasma emits visible and EUV radiation. Excitation of theXenon molecules causes the electrons to transition between their shellsto produce EUV photon radiation.

The EUV source 104 may produce radiation (photons) with a very shortwavelength, such as about 13 nanometers. In other embodiments, thephotons may have other wavelengths.

As an example, it may be desirable for the EUV lithography tool 100 todeliver about 50-120 watts of “clean” power to projection optics for athroughput of 80 wafers per hour.

Single Hemisphere Radiation Collection

FIG. 2 illustrates an example of radiation collectors 232 in a singlehemisphere. FIG. 2 is a cross-sectional side view of an apparatus 200which may be used in a Dense Plasma Focus (DPF) electric dischargesource. The apparatus 200 includes an anode 208, cathode 202 andinsulator 212. The apparatus 200 may be used with a buffer gas 206, suchas Helium, a source gas 214, such as Xenon, a foil trap 230, a grazingincidence collector 232 and a pump 234. The anode 208 may be coupled toa high voltage source 216, and the cathode 202 may be grounded.

Extreme ultraviolet (EUV) light sources, particularly laser producedplasma EUV sources, may have a finite power output that is radiated intoa large solid angle of up to about 4Π. Laser plasma sources tend to bevery small, e.g., about 300 microns in diameter. Due to their smallsize, a laser plasma source may be modeled as a quasi-point source.

The ability to collect the large solid angle light has been limited byseveral factors, including mirror fabrication, coating requirements, andthe large solid angle itself. Currently, near-normal incidencecollectors or grazing incidence collectors try to resolve thesechallenges to collect light. But the collectors are only in a singlehemisphere, as shown in FIGS. 1 and 2.

Dual Hemisphere Radiation Collection

The above challenges may be addressed by using collectors in bothhemispheres (e.g., 320 and 330 in FIG. 3) around the radiationsource/object (e.g., 306), as described with reference to FIGS. 3-7.Dual hemisphere radiation collectors may be implemented with a laserproduced plasma (LPP) source 104 of FIG. 1 or any other light sourcethat radiates light into a large solid angle up to about 4Π.

FIG. 3 is a cross-sectional view of a dual hemispherical collectorsystem 300, which may be used in a lithography illumination system, suchas the system 100 of FIG. 1. A radiation source 306 in FIG. 3 emitsforward radiation 312 in one hemisphere 320 and backward radiation 310in another hemisphere 330. The backward radiation 310 may be reflectedby a first collector (C1) 309 (also called a “surface,” “mirror” or“condenser”) at near normal incidence, i.e., almost perpendicular, e.g.,75-89 degrees. The first collector 309 may have a spherical oraspherical shape, i.e., elliptic or other conics.

A second collector 302 may have a grazing incidence, e.g., less thenabout 60 degrees, and may reflect forward directed radiation 312 fromthe source 306. Side 302B is a continuous bottom part of the secondcollector 302A. The second collector 302 may have an aspherical shape,.e.g., elliptic.

Light from both hemispheres 320 and 330 around the source 306 may befocused with a single reflection by the collectors C1, C2 309, 302towards an output point or x-z plane 308 with a “z” axis into the pageand an “x” horizontal axis. The collectors 302, 309 may be shaped, sizedand positioned to reflect light from the source 306 to the output pointor plane 308. Examples of values for FIG. 3 may be expressed as follows.For the first collector (C1) 309:

NA₁ (Numerical Aperture)=0.75, L₁=100 mm,

theta₁=sin⁻¹ (NA₁)→H₁=113.389 mm.

1/L₁+1/(a+c)=2/r₁

where “a” and “c” are variables to solve for an elliptic collector inFIGS. 3 and 4. “a” is a major axis of the ellipse 400 in FIG. 4. “b” isa minor axis of the ellipse 400. “c” is a focal point of the ellipse 400from the origin. r₁ is the radius of curvature of the asphericalcollector C1 309. r₂ is the radius of curvature of the ellipse 400 atpoint “a” in FIG. 4. If L₁, “a” and “c” are known, then the equationabove may be solved for r₁.

For the second collector (C2) 302:

(2c−L₄)tan[sin⁻¹ (NA₂)]=

(b/a) (sqr rt of (a−c+L₄)) (sqr rt of (2a−a+c−L₄))

c²=a²b² and

2(sqr rt of (c²+b²))=

(sqr rt of (H₄ ²+(2c−L₄)²))+(sqr rt of (H₄ ²+L₄))

with L₄=400, NA₂=0.25/4=0.0625,

theta₄→H₄

FIG. 4 illustrates a table of values for theta₄ and a, b, c, c/a, r₂,f₁=a−c and r₁. FIG. 4 also illustrates equations for an ellipse 400:

z=a(1−sqr rt of (1−x²/b²))

x=(b/a)(sqr rt of z)(sqr rt of (2a−z))

z=(x²/r₂)/(1+sqr rt of (1−(1+k) (x/r₂)²))

FIG. 5 is a cross-sectional side view of another embodiment of a dualhemispherical collector system 500, which may be used in a lithographyillumination system. In FIG. 5, a light source 506 generates light 510backward in a first hemisphere 530 to a first collector (C1) 509 (alsocalled “surface,” “mirror” or “condenser”) and generates light 512forward to a set of second collectors (C2) 502, 504, 506.

The first collector 509 may have a spherical or aspherical shape, i.e.,elliptic or other conics as well. The first collector 509 focuses light510 back through a region around the source 506, and the light undergoesreflections from the set of second collectors (C2) 502, 504, 506. Thus,light 510 from the backward direction undergoes reflections from a nearnormal incidence collector 509 and then grazing incidence collectors502, 504, 506.

For the first collector (C1) 509:

1/L₁+1/L₂=2/r₁→r=L₁

FIG. 5 also shows the cross-sectional tops 502A, 504A, 506A and bottoms502B, 504B, 506B of the set of mirrors or grazing incidence collectors502, 504, 506. The collectors 502, 504, 506 may form a rotation aboutthe optic axis 516. From point 508 looking back at the source 506, thecollectors 502, 504, 506 may appear to be three concentric rings (havingthe same center). The collectors 502, 504, 506 may be symmetric. Thecollectors 502, 504, 506 may be held together by one or more spokes 514.

Each collector 502, 504, 506 may be a “Wolter” collector, which may beknown to those of ordinary skill in the art. For example, the collector502 may have a first part 503B closer to the source 506 that isgenerally hyperbolic in shape and a second part 503A farther from thesource 506 that is generally elliptic in shape. The collectors 502, 504,506 collect and reflect forward directed radiation 512 to a point orplane 508.

A Wolter collector includes two conic mirror segments 503A, 503B used toform a grazing incidence imaging optic. The total reflectance loss forsmall and similar angles of incidence on the two mirrors may beapproximately equivalent to the loss from a single larger reflectionangle design.

FIG. 6 illustrates an example of a hyperbola 602 and ellipse 600 formodeling a shape of the Wolter collector 502. The hyperbola 602 may havea real focus (focal point) at f1 and a virtual focus at f2. The source506 in FIG. 5 may be positioned at the real focus f1 of the hyperbola602. The ellipse 600 may have a first focus at f2 and a second focus atf3. Thus, the first focus of the ellipse 600 is at the virtual or secondfocus f2 of the hyperbola 602. The output point or image plane 508 maybe positioned at f3, which is the second focus f3 of the ellipse 600.

A light beam 604 from the source 506 reflects off the collector 502A tothe point/plane 508. Another light beam 606 reflects off the hyperbolicpart 503B and then the elliptic part 503A of the collector 502A to thepoint/plane 508.

The collector systems 300, 500 collect light in a large solid angle fromquasi point sources 306, 506, such as a laser produced plasma sourceused in extreme ultra-violet (EUV) lithography. Common key aspects ofthe systems 300, 500 may include:

1. A collector system 300, 500 composed of reflective surfaces 302, 304,309 and 502, 504, 509 in both forward and backward hemispheres 320, 330and 520, 530 around an object or light source 306 and 506.

2. Reflective surfaces 302, 304, 309 and 502, 504, 509 of the systems300, 500 may be arranged to project light from the sources 306, 506 ontoa point or a plane 308, 508 along the x-z axes.

The systems 300, 500 may have light collection subtending a solid anglegreater then 2Π in lithography illumination. Each mirror may beconfigured and coated separately.

Using both hemispheres 320, 330, 520, 530 may achieve a largercollection angle and more collected radiation.

FIG. 7 illustrates a method of using a dual hemispherical collectorsystem. The method includes providing a first reflective surface in afirst hemisphere of a radiation source and a second reflective surfacein a second hemisphere of the radiation source at 700; reflectingradiation from the source in the first hemisphere to the secondhemisphere at 702; and reflecting radiation from the source in thesecond hemisphere to a pre-determined plane at 704.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the application. Accordingly, other embodimentsare within the scope of the following claims.

1. A lithography system, comprising: a source that generates radiationbackward in a first hemisphere and forward in a second hemisphere; afirst reflector located in the first hemisphere that collects andreflects a first portion of radiation from the source to a target plane,wherein the source is located between the target plane and the firstreflector; and a plurality of second reflectors located in the secondhemisphere between the target plane and the source and offset from anoptic axis between the target plane and the source that collects andreflects at least a second portion of radiation from the source to thetarget plane, each second reflector comprising a first reflector portionand a second reflector portion in different reflector geometries.
 2. Thesystem as in claim 1, wherein each second reflector is a Woltercollector.
 3. The system as in claim 1, wherein the plurality of secondreflectors is symmetrically arranged around the optic axis between thetarget plane and the source.
 4. The system as in claim 1, wherein thesource is a laser produced plasma (LPP) source.
 5. The system as inclaim 1 wherein the first reflector portion is closer to the source andhas a hyperbolic shape.
 6. the system as in claim 1 wherein the secondreflector portion is farther from the source and has an elliptic shape.7. The system as in claim 1 wherein the plurality of second reflectorscomprises three concentric rings.
 8. The system as in claim 1 whereinthe first reflector comprises a near normal incidence collector.
 9. Thesystem as in claim 1 wherein the plurality of second reflectorscomprises grazing incidence collectors.
 10. The system as in claim 1wherein the first reflector has a spherical shape.
 11. The system as inclaim 1 wherein the first reflector has an aspherical shape.
 12. Thesystem as in claim 1 wherein the first reflector has an elliptic shape.13. The system as in claim 1 wherein the source is an electric dischargesource.
 14. The system as in claim 1 wherein the source generatesradiation into a large solid angle up to 4Π.
 15. The system as in claim1 wherein the plurality of second reflectors is symmetric around theoptic axis.
 16. The system as in claim 1 wherein the plurality of secondreflectors is held together by one or more spokes.